Without a doubt, the suspension and brakes make the biggest difference on the track. Not even having to think about braking while on the track is such a relief. Changing to track pads literally takes 5 minutes per corner.

LSD is fantastic for balancing the car mid-corner. Higher HP cars would benefit even more as I'm only making about 270hp with my 330.

The seats - my god the seats make such a difference in feeling what the chassis is doing I can't even begin to describe it. It is mind-blowing!
I had to go with the Jaffster shift knob because I'm sitting about 2-1/2" lower in my car than with my stock sport seats. My elbow still hits the center console door (I'm going to recover it removing the foam to make it a lower profile). They are very comfortable on the street too, not being race shells and all. So, they're not the lightest seats either. The two of them net out to be 30lbs lighter than stock. They have lumbar support too.

The main reason I listed the CF hood, trunk and light weight battery is with 3/4 of a tank of gas I weighed my car. It is 3,270lbs. That means I dropped about 180lbs off my curb weight. My car feels very nimble.

R-comps are r-comps. They are the crack of tires. What else needs to be said? I added 5mph down the front straight just by adding the r-comps.
My car is setup to be very stable and doesn't do anything drastic, even if I overcook a turn.